1. Field of the Invention
The present invention relates generally to the formation of contacts in integrated circuits, and more particularly to a method of forming bit line contacts in dynamic random access memory chips.
2. Description of the Related Art
Integrated circuits, also commonly referred to as semiconductor devices, are formed of various electrically conducting, semiconducting and insulating materials. Silicon, in single crystal, amorphous or polycrystalline form, is the most commonly used semiconductor material. Silicon can be made electrically conductive by adding impurities, commonly referred to as doping. Through a series of doping, deposition and etch steps, electrical devices are formed and interconnected to produce the integrated circuits.
Dynamic random access memory (DRAM) circuits include arrays of memory cells, each of which includes two basic components: a field effect transistor (FET) and a storage capacitor. Typically, a semiconducting substrate is doped to produce active areas of an access transistor, one of which is connected to the lower or storage electrode of the capacitor. The other active area and the transistor gate electrode are connected to external connection lines, namely digit or bit lines and word lines or rows. The top or reference electrode of the capacitor is connected to a reference voltage. DRAM arrays thus include a transistors, capacitors and contacts to interconnecting lines.
It is advantageous to form integrated circuits with smaller individual elements so that as many elements as possible may be formed in a single chip. In this way, electronic equipment becomes smaller and more reliable, assembly and packaging costs are minimized and circuit performance is improved. In particular, denser device packing leads to faster and more efficient circuit operation. Despite the focus on continued miniaturization, the storage capacity of the cell capacitor must generally remain above a minimum level to ensure reliable operation (low error rates). Consequently, the development of faster and more powerful DRAM chips focuses in large part on maintaining capacitance despite shrinking available chip area for each memory cell.
One way in which capacitance has been increased has been to increase the surface area of the capacitor electrodes by creating three-dimensional folding structures to which the electrodes conform. When the capacitor is formed above the transistors, they are known in the industry as “stacked” capacitors. Stacked capacitors advantageously demonstrate high capacitance per unit of occupied chip area (“footprint”), high reliability and simple process integration relative to other capacitor designs.
As noted, in the process of fabricating a DRAM chip, electrical connections must be made to the transistor active areas. The active areas, which are also known as source and drain regions, are discrete doped regions in the surface of the semiconductor substrate. As the size of the DRAM is reduced, the size of the active areas and the corridors available for contacts to reach the active areas are also reduced. At the same time, insulating materials must be maintained to effectively isolate the contacts from the transistor and capacitor components. Accordingly, the width of bit line contacts, and other integrated contacts generally, must shrink as device packing density increases.
Unfortunately, while contact width continually decreases, contact height cannot decrease proportionately. Rather, the contact height is defined by the thickness of the interlevel dielectric (ILD) which separates the two levels in the circuit, such as the substrate and higher wiring levels. The ILD thickness, in turn, must be maintained to minimize the risk of short circuits, as well as to prevent interlevel capacitance, which can tie up electrical carriers and slow signal propagation.
Relative increases in contact height is particularly acute in DRAM circuit designs which incorporate stacked capacitors. As noted, capacitance is proportional to the surface area, which depends on both height and width. In order to maintain the same or higher level of capacitance from generation to generation, the capacitor height must remain the same or even increase as device spacing decreases. In bit-over-capacitor (BOC) designs, the bit line contact increases in height along with the capacitor.
While contact width decreases and contact height is essentially maintained, the aspect ratio (defined as the ratio of height to width of a contact) of contacts continues to increase. In general, therefore, each successive generation of integrated circuits incorporates contacts of higher aspect ratios, and this is particularly true for certain DRAM circuit designs. As is well known in the art of integrated circuit fabrication, high aspect ratio contact vias are very difficult to fill without forming keyholes.
One partial solution to this problem is to reduce the dielectric constant of the ILD. A lowered dielectric constant enables thinning ILDs for a given tolerable parasitic capacitance, and consequently lowers the aspect ratio. This solution, however, can only be carried so far before the dielectric loses its insulating qualities, or the risk of short circuits through the thinned ILD becomes too high. Moreover, this solution does not address the high aspect ratios of bit line contacts necessitated by high stacked capacitors in bit-over-capacitor DRAM circuit designs.
Accordingly, a need exists for more effective methods of forming contacts between levels in integrated circuits.